1. Field of the Invention
The invention relates to the field of digital amplifier design and more particularly to digital pulse width modulated (PWM) amplifiers powered by sources having power source ripple which is compensated for by the amplifier design.
2. Description of Prior Art
PWM audio power amplifiers have become important in the last decade due to their high efficiency and wide dynamic range. The input audio signals for such power amplifiers can be analog or digital. Digital audio has found increasingly wide applications due to the success of digital storage media and the advances of digital signal processing technology. A digital-PWM power amplifier for audio amplification is preferred, since it eliminates the need for an extra digital-to-analog converter, associated sampling and hold circuitry, and an analog low-pass output filter with a very sharp cutoff as are needed in a conventional digital-analog PWM amplifier. By using a digital-PWM power amplifier the high fidelity of the pulse code modulated (PCM) audio signal can be maintained all the way to the analog loud speakers.
Conventional digital-PWM power amplifiers rely on high-precision DC power sources to obtain high fidelity. These power amplifiers transmit any ripple in the power source to the output, thereby causing signal distortion at the speaker. Consequently, high precision DC power sources are required in order to obtain high signal-to-noise ratios. These DC power sources usually comprise a rectifier, low-pass filter and a regulator. The higher the desired precision for the power amplifier, the larger the capacitance or inductance required in low-pass power filter and the regulator. This then requires large, expensive and often nonlinear discrete components.
The reduction of distortion in power amplifiers is a well recognized need and several approaches have been devised in an attempt to realize it. Swanson et al., "Method and Apparatus for Reducing Distortion in Amplifiers," U.S. Pat. No. 4,737,731 (1988) shows an analog circuit for reducing distortion due to noise on the DC power supply line and amplifiers. In the amplitude modulator of FIG. 2, divider 26 is employed to divide the incoming audio frequency signal from source 12 by V1, a signal which is proportional to that provided by the DC source 24. The output of divider 26 is then amplified by high powered audio amplifier 14.
Swanson's express purpose for including divider 26 is to eliminate distortion caused by noise on the DC power line, one component of which is the AC ripple resident on that line. Swans does not contemplate the use of switched amplifiers and it implements the distortion reducing feedback in analog circuitry.
Swanson, "Pulse Duration Amplifier System Having Distortion Reduction," U.S. Pat. No. 5,216,376 (1993) shows in FIG. 1 an amplifier system having a feedback voltage Vs from the outputs of pulse duration modulation amplifiers 10, 12, 14 and 16 which are provided to divider 96. The input audio voltage Va is combined by divider 96 with the feedback voltage Vs such that the voltage of the signal fed into PDM amplifiers 10, 12, 14 and 16 is ratio Va/Vs. The effect of this division is to cancel the variations of the system's output signal due to the B+ supply line voltage variation, as well as variations due to fluctuating impedance when the various power amplifiers are turned on.
Swanson, "Power Amplifier System Having Improved Distortion Reduction," U.S. Pat. No. 5,132,637 (1992), is cumulative in his teaching although it shows the same principle used in a different circuit. In this case, feedback is provided to divider 254 from a plurality of amplifiers PA.
Covill, "AM Pulse Duration Modulator," U.S. Pat. No. 4,605,910 (1986), shows a PWM circuit including an improvement to reduce harmonic distortion caused by the AC ripple voltage on the DC voltage supply line. The modulator diagram to FIG. 2 combines a reciprocal of the supply voltage with the modulating signal. Multiplier 20 and operational amplifier 18 together form a divider circuit which divides a multiple of the input modulating signal by supply voltage S. The signal which controls modulator 17 is, therefore, substantially independent of the fluctuations in the supply voltage. The switching modulating signal appears at the output port of the amplifier. This switching modulating signal, when used to switch a pulse duration modulator, provides a signal at the output of the modulator which is independent of the supply voltage.
Malec, "Distortion Correction for an Amplifier System," U.S. Pat. No. 5,150,072 (1992), shows in FIG. 1 a divider 39 which receives two inputs. One is input audio signal 12 with a DC level added to it by analog processing circuit 33. The other input is a sample voltage V1 originating from the line voltage source 19, the ac proponent of which has been optimally phase adjusted by correction circuit 41. Analog divider circuit 39 combines these inputs such that the input audio signal is divided by the sample voltage V1. The output is then passed to a digitizer 16 through an A-to-D converter 37 and then provided to amplifiers PA1-PAn at their inputs. In this manner, distortion due to ac components and the voltage source 19 are cancelled.
Holmes, Jr., "AC Amplifier with Automatic DC Compensation," U.S. Pat. No. 5,115,205 (1992), was cited for showing incorporation of the wave shaping feedback system to control the input to a pulse width modulator. As best depicted in FIG. 2, output ports A and B of pulse width modulator 26 drive analog switches 54 and 56 such that voltages V+ and V- are alternately fed back via junction 58 to the input. This feedback signal combines with the signal from sine wave oscillator 12 to feed input 16 of air amplifier 14. The effect of the feedback scheme is to reduce or eliminate the net DC voltage applied to output transformer T1. Nevertheless, any divider function operated in the analog domain has a very limited accuracy and dynamic range.
In conventional PWM approaches, the original signal is converted into pulses whose duration is proportional to the original signal. In the digital domain, this can be achieved by taking the sample amplitude of the signal and using its value to determine the length of a pulse of constant amplitude. The counter is loaded with required data volume and clocked until empty. When the counter starts, the output is set and when the counter finishes, the output is reset. For a 16-bit word arriving at the rate of 44.1 kHz as in a music compact disc, a counter rate of 2.89 GHz is required. Various prior art schemes have been used to reduce the clock rate to a practical range while maintaining the original resolution.
The prior art concept of noise shaping is to express a signal with fewer bits than the given audio band quality would require. The requantization error is modified so that the noise flow in the audio band is reduced at the expense of noise elsewhere.
A noise shaping digital-PWM power amplifier typical of the prior art is shown in FIG. 1. The amplifier, generally drawn by reference numeral 10, includes an interpolator 12, a noise shaper 14, a pulse width modulator 16, a push-pull converter 18, a precision DC power source 20 and a low-pass power filter 22. The digital input signal has a resolution of b bits and a carrier frequency of f.sub.c. The input signal is interpolated to a sampling rate by L times. The interpolated signal of b bits with a frequency Lf.sub.c is passed to noise shaper 14 and the noise shape signal of b' bits is obtained with a lower resolution, i.e., b'&lt;b, which is then converted to a pulse width modulated signal by modulator 16. The pulse width modulation is typically implemented by a counter clock with a frequency proportional to 2.sub.b' L.
A typical prior art noise shaper is illustrated in FIG. 2. The signal X on input line 24 has b bits and the output signal on output line 26 has b' bits. FIG. 2 diagrammatically illustrates the insertion of noise shaping error, e.sub.ns and requantization error, e.sub.rq. To be shown at the square of the noise transfer function has the same shape as the power spectral density of the output error, e.sub.ns, assuming that the requantization error closely approximates white noise. If element 28 is a high-pass filter, the requantization noise in the audio band will be attenuated by the noise transfer function. With this noise shaping scheme, it is possible to reduce the counter clock frequency substantially.
For example, using an 8 times over sampling rate and a 4th order noise transfer function, namely assuming that filter 28 is a 4th order high-pass filter, an original 16-bit signal can be reduced to 8 bits, and therefore implemented with a counter clock frequency of 90.3 MHz while the output signal-to-noise ratio in the audio band remains the same. This scheme, however, requires a high precision DC power source as shown in FIG. 1 in order to achieve high fidelity.
One prior art scheme to avoid the use of high precision DC power sources called for a feed forward path using a buck converter for ripple reduction. The power source voltage, v.sub.s, is sensed to modulate the amplitude of the sawtooth in the ripple. For a constant input signal v.sub.i, when the power source voltage steps up, the sawtooth becomes steeper and the duty ratio changes in an attempt to reject the input voltage perturbation. The slope of the sawtooth is proportional to the power source voltage. However, when the power source contains ripple, the so-called "sawtooth" is no longer linear in the manner just described. The workable dynamic range of the power source voltage is limited since the height of the sawtooth is limited by its power supply. FIG. 3 is a wave diagram showing the power source voltage on line 30, the sawtooth wave form of the buck converter on line 32 and the resulting PWM signal on line 34.
What is needed is some type of circuit or approach in which low cost unregulated power supplies, having a substantial ripple content, can be used with a digital PWM amplifier without the ripple distortion caused by the unregulated power supply appearing in the output of the digital PWM amplifier.
Therefore, what is needed is a digital-PWM power amplification technique featuring noise and ripple shaping for audio power amplification.